Dynamic micro testing over more than 5 decades of strain rates

Abstract

Based on the combined experience in micro testing [1] and testing under crash and impact loads [2] at Fraunhofer Institute for Mechanics of Materials IWM, a new testing system was developed for micro sample testing over a wide range of strain rates. The aim was to realize a testing facility allowing the investigation at small sample sizes, taken out of sheet materials or components for local material characterization over a large range of strain rates [3]. The first application and reference testing series was performed at a dual phase steel HCT490X rolled to a thickness of 0.56 mm with sample main dimensions of 3.5 mm x 3.5 mm. These tests were performed in parallel with mini tensile samples (Fig. 1, right) at a conventional fast servo hydraulic testing machine (VeryHighSpeed, VHS) at com parable strain rates, ranging from 0.001 s-1 up to 1000 s-1 [3] These parallel testing series showed up the benefit of small micro samples for strain rate dependent investigations. The micro sample tests showed comparable stress strain reactions with significant smaller oscillations in the force signal than the mini sample test. For this strain rate range the oscillations in the force signals were significantly improved compared to the conventional tests. For ongoing tests the samples were prepared out of different polymer materials including long fiber reinforced thermoplastics (LFT). These tests focus on the investigation in homogenous local material reaction to different loading velocities. For these tests, the sample preparation was already a challenge that was overcome using micro water jet cutting with a well-defined process chain and sample finishing. The test series are subject of actual investigations. In this paper an overview of the actual status of dynamic micro testing is given